Download MASSACHUSETTS INSTITUTE OF TECHNOLOGY Becoming Familiar with the hardware and software.

MASSACHUSETTS INSTITUTE OF TECHNOLOGY
6.071 Introduction to Electronics, Signals and Measurement
Spring 2006
Becoming Familiar with the hardware and software.
The laboratory equipment for our class are comprised of a set of hardware and software
technologies which are tightly integrated, yet very flexible in their use, and will give us
the opportunity to investigate in a practical way all concepts introduced in the class.
The hardware part of our instrument is shown below.
Courtesy of National Instruments. Used with permission.
The software is based on the National Instruments LabVIEW program. In a simple phrase
the LabVIEW software gives us the opportunity to 1) control the hardware, 2) generate
and manipulate the various electrical signals of the system and 3) create effective
presentation of the analyzed data.
The document with the manual of our system is located at the MIT server.
6.071 Spring 2006, Chaniotakis and Cory
1
Let’s Play Around
During this first encounter with our system we will go through a number of exercises
designed to help us understand the operation and the overall characteristics of the
system.
Exercise 1. Turning on the System, launching the program.
The power switch for the system is located on the rear of the device. The prototyping
board is powered by the switch on the left of the front panel. The first thing to check is
the status of the system indicated by the three green leds on the lower left hand corner of
the prototyping board. If any of these lights is off the appropriate fuse must be replaced.
Courtesy of National Instruments. Used with permission.
6.071 Spring 2006, Chaniotakis and Cory
2
Courtesy of National Instruments. Used with permission.
Once the system is powered we can turn our attention to launching the software that
drives and controls the device.
The ELVIS software can be launched from the icon named Elvis on your desktop.
The following information will appear.
Courtesy of National Instruments. Used with permission.
6.071 Spring 2006, Chaniotakis and Cory
3
The initializing progress bar indicates that communication is established among the
various hardware components (PC to PCI card, PCI card to ELVIS).
Now we are ready to start. First let us take a close look at the prototyping board. A
schematic of it is shown below.
Courtesy of National Instruments. Used with permission.
The main section of the prototyping board is made up of the power traces labeled with the
red and blue lines and the space for component placement sandwiched between the power
traces. For effective circuit building on the prototyping board we should adhere to the
following guidelines.
1. Make sure the system power is off while you are building your circuits.
2. Establish power traces and test them. For example if you will be building a circuit
that needs +5V and ground then first establish the +5V as one of the red traces
and the ground as the adjacent blue by connecting them to the power supply
(region 6). Test the actual value of the voltage at the power traces with the
multimeter or the oscilloscope (more on this later).
6.071 Spring 2006, Chaniotakis and Cory
4
3. Turn the system back off and start building your circuit.
4. Keep your wires short and do not run wires over components
5. Build your circuit is sections and test each section before proceeding.
Exercise 2.
For this exercise we will use the Digital Multimeter (DMM) feature to measure the value
of some electronic signals and electronic components.
We may also use the DMM to measure the potential (voltage) at some point of a circuit.
Well, here is a question that you may very well ask. Does the measurement itself affect
the result? The answer to this question is YES if you are not careful or if the measuring
device is not appropriately designed. The two electronic engineering phrases to these
questions are “proper grounding” and “impedance matching”. We will explore these
issues as well in a few days. For now let’s just keep asking the questions.
Our device has some power supply capabilities and we will use these to create and
measure the voltage generated by the supply. Notice that in section 6of the prototyping
board there are two regions labeled DC Power Supplies and Variable Power Supplies.
First lets measure the +5 Volts. To do this connect the +5V to the VOLTAGE HI and the
Ground to VOLTAGE LO. By selecting the appropriate Function on your DMM
instrument you should be able to perform this voltage measurement. Do the same for the
+15V and the -15V.
Next let’s connect to the Variable Power Supply and measure the Voltage. We will
change the actual voltage of the Variable Power Supply manually. We can do this by
switching the Variable power supply to the manual mode with the switch located on the
front of the unit. Measure the voltage for both the Supply+ and Supply- .
Next let us measure the value of a resistor.
Just a brief note on how the actual measurement of resistance is accomplished: In order to
determine the resistance value the device measures an electrical signal, a voltage in this
case, and it then relates the value of the voltage to the resistance. (A big lesson here:
NEVER MEASURE COMPONENT VALUES WHILE THE COMPONENT IS
CONNECTED TO THE CIRCUIT, OR EVEN WORSE WHEN THE CIRCUIT IS
POWERED.)
Select a resistor and connect it to the pins labeled CURRENT HI and CURRENT LO
located in the DMM section of your protoboard. Set your DMM to the resistnace
measuring mode denoted by Ω. Play with the range have fun.
6.071 Spring 2006, Chaniotakis and Cory
5
Now lets measure the value of a capacitor. (Yes I know we haven’t learned about these
components yet but so what?). Again connect the capacitor to the pins labeled
CURRENT HI and CURRENT LO and experiment by changing the range.
Next we will do an experiment to measure the temperature in the room. For this you will
use a thermistor, which as we just learned is a temperature dependent resistor.
Connect the thermistor to the pins labeled CURRENT HI and CURRENT LO.
What is the resistance you are measuring?
Does it fluctuate or does it change monotonically?
Once the value stabilizes read and record the resistance.
In order to determine the corresponding temperature look up the resistance versus
temperature curve included in today’s notes.
What is the room temperature?
As the last experiment for today touch the thermistor and see the effect in the resistance
reading.
Does the resistance increase or decrease with temperature?
How close to your body temperature (370C) can you reach by holding the thermistor in
your fingers?
Close the Digital Multimeter instrument window.
6.071 Spring 2006, Chaniotakis and Cory
6
Thermistor. A Thermoelectric transducer
Thermistors are non-linear temperature dependent resistors with a high resistance
temperature coefficient. They are advanced ceramics where the repeatable electrical
characteristics of the molecular structure allow them to be used as solid-state, resistive
temperature sensors. This molecular structure is obtained by mixing metal oxides
together in varying proportions to create a material with the proper resistivity.
Two types of Thermistors are available: Negative Temperature Coefficient (NTC),
resistance decreases with increasing temperature and Positive Temperature Coefficient
(PTC), resistance increases with increasing temperature. In practice only NTC
Thermistors are used for temperature measurement. PTC Thermistors are primarily used
for relative temperature detection.
In this class we will use an NTC thermistor. The temperature versus resistance data of our
thermistor is shown on the table and figure below.
Resistance multiplier 10k
NTC Thermistor
40000
35000
Resistance (Ohm)
30000
25000
20000
15000
10000
5000
0
0
10
20
30
40
50
60
70
Temperature (Celcius)
6.071 Spring 2006, Chaniotakis and Cory
7
For convenience we would like derive a mathematical expression which describes the behavior of
the device. The Steinhart and Hart equation is an empirical expression that has been determined
to be the best mathematical expression for resistance temperature relationship of NTC
thermistors. The most common form of this equation is:
1
= a + b(ln R ) + c(ln R )3
T
Eq. 1
Where T is in Kelvin and R in Ω. The coefficients a, b, c are constants which in principle are
determined by measuring the thermistor resistance at three different temperatures T1 , T2 , T3 and
then solving the resulting three equations for a, b, c .
1
= a + b(ln R1 ) + c(ln R1 )3
T1
1
= a + b(ln R2 ) + c(ln R2 )3
T2
1
= a + b(ln R3 ) + c (ln R3 )3
T3
The parameters a, b, c for the thermistors provided with the lab kit in the temperature
range between 0-50 degrees Celcius are:
a = 1.1869 × 10−3
b = 2.2790 × 10−4
c = 8.7000 × 10−8
The Steinhart and Hart equation may be used in two ways.
1) If resistance is known, the temperature may be determined from Eq. 1
2) If temperature is known the resistance is determined from the following equation:
1/ 3
1/ 3
⎡⎛
α⎞
α⎞ ⎤
⎛
R = exp ⎢⎜ β − ⎟ − ⎜ β + ⎟ ⎥ ,
2⎠
2 ⎠ ⎦⎥
⎝
⎣⎢⎝
6.071 Spring 2006, Chaniotakis and Cory
8
Where, α =
1
2
Τ and β = ⎛ b ⎞ + ⎛ α ⎞
⎜ ⎟ ⎜ ⎟
c
⎝ 3c ⎠ ⎝ 4 ⎠
a−
The non-linear resistance versus temperature behavior of the thermistor is the main
disadvantage of these devices. However, with the availability of low cost microcontroller
systems the non linear behavior can be handled in software by simply evaluating the
Steinhart and Hart equation at the desired point.
dR
, varies with temperature. For our thermistor the
dT
sensitivity as a function of temperature is shown on the following figure.
The sensitivity of the thermistor,
NTC Thermistor Sensitivity
0.00
0
10
20
30
40
50
60
70
80
-200.00
-400.00
dR/dT (Ohms/Degree)
-600.00
-800.00
-1000.00
-1200.00
-1400.00
-1600.00
-1800.00
-2000.00
Temperature (Celcius)
Note that the thermistor sensitivity decreases with increasing temperature. This is the
primary reason for the small temperature measuring range of thermistors. Notice however
that in temperature range of interest to biological and most environmental applications
the sensitivity is greater than 100Ω/degree Celsius which results in the design of sensitive
and robust systems.
6.071 Spring 2006, Chaniotakis and Cory
9